Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling parts in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Reaction (1).CH4+H2O⇄CO+3H2  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. The steam reforming reaction is endothermic (the heat of reaction (1) is about 9 kcal/mol of methane), requiring the expenditure of large amounts of fuel to produce the necessary heat for the industrial scale process. Another drawback of steam reforming is that for many industrial applications, the 3:1 ratio of H2:CO products is problematic, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
The catalytic partial oxidation (“CPOX”) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial or direct oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Reaction (2):CH4+1/2O2⇄CO+2H2  (2)
The H2:CO ratio for this reaction is more useful for the downstream conversion of syngas to chemicals such as methanol or other fuels than is the H2:CO ratio from steam reforming. In addition, the CPOX reaction is exothermic (−8.5 kcal/mol), in contrast to the endothermic steam reforming reaction. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes than is possible in a conventional steam reforming process. All of these factors lower the cost for the conversion of methane or natural gas and make the CPOX reaction much more attractive for commercial use.
After syngas is obtained through the above-mentioned process, it is then converted to higher hydrocarbons (paraffins in the range of C5 to C20) by a variety of liquid hydrocarbon synthesis processes. One such process is via Fischer-Tropsch (FT) synthesis using a metal catalyst, through Reaction (3).CO+2H2→1/n(CnH2n)+H2O  (3)
There are primarily two broad types of catalyst used in FT synthesis: Fe-based catalysts and Co-based catalysts. The literature is replete with discussions of these catalysts and their varying compositions. Cobalt catalysts are generally considered a better match for the conversion of methane or natural gas derived syngas. For these catalysts, sulfur and oxygen are poisons that must be removed from the feedstock. It is known that sulfur will permanently deactivate a cobalt catalyst when present in concentrations of 50 ppb or greater. It has recently been discovered that oxygen can temporarily or even permanently deactivate a cobalt based catalyst when present even in very low amounts. The term “non-toxic” will be used herein to describe a syngas stream having an oxygen concentration at or below a level that is acceptable for whatever application the syngas is to be used downstream, including catalytic FT synthesis.
The concentration of oxygen in syngas is typically determined by the reactive process used to derive the syngas. Traditional methods for producing synthesis gas, including steam methane reforming and auto-thermal reforming, are characterized by relatively long periods of reactant exposure to the catalyst beds. Long exposure times allow the reaction to consume any unconverted oxygen that may remain in the syngas.
Due to the commercial importance of syngas, there is a continuing effort to maximize the efficiency of syngas and liquid hydrocarbon productivity by developing new methods for preparing syngas. Smaller catalyst beds and shorter exposure times characterize some of these new methods. These new reactors are commonly referred to as short contact time reactors (SCTR). There are several advantages to short contact time reactors, i.e., increased productivity due to higher space velocities, smaller volumes of catalyst needed, smaller catalyst beds, etc.
In spite of the benefits, there is a greater opportunity for oxygen to pass through the reactor unconverted. One such opportunity for oxygen to pass through the reactor unconverted is due to the increased velocity of the gas through a thin fixed bed reactor. The traditional methods mentioned above generally have gas hourly space velocities (GHSV, the standard volume of gas flow through per volume of catalyst per hour) near 4,000 per hour, whereas the new SCTR designs can have GHSV as great as 1,000,000 per hour or higher. Oxygen breakthrough can result due to the millisecond residence time of the reactants. The higher space velocities also force gas to pass through “short cut channels” the catalyst bed or fractures in the insulation refractory before it can be exposed to the active catalyst resulting in increased concentrations of unconverted oxygen in the syngas product.
In addition, it is well known in the art that catalysts “age” with time and use. Aging occurs for a variety of reasons including coke deposition, poisoning, etc. The more aged a particular catalyst is the less efficient the catalyst is at initiating reaction, i.e., less activity it has. As the catalyst ages more oxygen may pass through the bed unconverted.
For at least these reasons, some oxygen is able to pass through a syngas reactor without being converted. This increase in the unconverted oxygen concentration can lead to a decrease in efficiency of the downstream Fischer-Tropsch process due to oxidation or poisoning of the catalysts. Hence, in natural-gas derived syngas, especially those obtained from short contact time selective oxidation processes, there exists up to 0.5 (vol.) % oxygen, which can deactivate FT catalysts within several hours. Frequent regeneration of FT catalysts not only increases the difficulty of operation, but also significantly increases the associated costs.
Therefore, it is desired to decrease the oxygen concentration of the syngas by providing a method and apparatus for removing oxygen that remains in the syngas before the syngas is used in any downstream process, particularly a Fischer-Tropsch reaction.